1. Field of the Invention
The present invention relates to the operation of balanced antennas driven by balanced amplifiers, and more particularly relates to methods and systems for increasing output power efficiency of balanced amplifier/antenna combinations.
2. Description of the Prior Art
The use of class B power amplifiers, in a push-pull balanced configuration, to amplify a radio frequency (RF) signal, prior to the signal being transmitted into free space via an antenna, is well known in the prior art. Class B power amplifiers, whose output signal is a linear function of the input signal over 50 percent (180 degrees) of the input wave, offer the potential for very high power efficiency. It is also well known that this power efficiency, defined as the ratio of the amplifier RF output power to the amplifier dc input power, may reach a theoretical limit of 78.5 percent. Power amplifiers designed using metal semiconductor field effect transistors (MESFETs), high electron mobility transistors (HEMTs) or bipolar junction transistors (BJTs) may provide such theoretical efficiency.
However, due to the inherent non-linearities of these transistor devices, the output signal of a class B power amplifier suffers from significant harmonic distortion. This distortion takes the form of harmonic components, these components being present in the output voltage and current signals associated with the transistor devices. Class B push-pull operation seeks to eliminate these unwanted harmonic frequency components, particularly second harmonic load voltages, by summing them out of phase in order that they may cancel one another. In a conventional class B push-pull amplifier, this cancelling effect is accomplished by using a 180 degree output combiner wherein the second harmonic load voltages of each of the power transistors are summed and thus eliminated.
It is to be noted that if second harmonic load voltages generated by the push-pull class B power amplifiers are not properly terminated, this can result in the amplifier efficiency being degraded, thereby limiting the maximum level of efficiency to approximately 64 percent. Further, while the output combiner eliminates second harmonic components, the output combiner is also the source of significant dissipative power loss. Particularly, at microwave frequencies, the presence of the output combiner is responsible for degrading the amplifier efficiency by as much as 25 percent, thus limiting the ultimate power efficiency of the amplifier to approximately 59 percent. Accordingly, such power efficiency degradation, combined with this loss associated with the output combiner, can result in dropping the ultimate power efficiency of the amplification stage below 50 percent.
In addition, conventional systems for driving antennas also include several other necessary components. For instance, an impedance matching network is often required between the push-pull class B power amplifiers and the output combiner in order to match impedances of the amplifiers and the combiner. Further, a balun must often be used to convert the unbalanced feed coming from the output combiner into a balanced load such as is associated with a balanced antenna. Still further, the conventional systems may include additional harmonic filtering in the form of a harmonic filter placed between the output combiner and the balun. However, disadvantageously, all of these components have some degree of inherent dissipative power loss associated with them. Therefore, while they are necessary components in the conventional system, they also serve to degrade the high power efficiency level offered by the push-pull class B power amplifiers.
Accordingly, there is a need for a system and method which increases the power efficiency of a balanced amplifier/antenna combination by eliminating the dissipative power losses, as well as other sources of efficiency degradation, associated with the abovedescribed conventional arrangement.